The present invention relates to a photosensitive or photocurable resin composition and a planographic printing plate prepared with the resin composition as the photosensitive material. In particular, the present invention relates to a photosensitive resin composition suitable for the development with an aqueous alkaline development solution when it is used as the photosensitive material for the preparation of a planographic printing plate. More particularly, the invention relates to a photosensitive resin composition capable of forming image areas with excellent mechanical strengths and ink-receptivity on a planographic printing plate.
In the prior art, there has been known a process for the preparation of a planographic printing plate in which a base plate is coated with a photosensitive diazo resin such as a condensation product of 4-diazodiphenylamine and formaldehyde followed by exposure to light through a negative transparency to photocure the resin in the exposed areas and development by dissolving away the uncured resin with water into a negative type planographic printing plate. Several kinds of diazo resins are proposed as suitable for use in the above described plate making process (see, for example, Japanese Patent Publication Nos. 38-8907 and 38-11365).
One of the problems in these diazo photosensitive resins is that the mechanical strengths of the photocured layers of the resin in the areas exposed to light is insufficient when the resin is used as such. Proposed means as a remedy of the above defect of the diazo resins include providing a reinforcement overcoating layer on the photocured layer of the diazo resin by coating with a lacquer and improving the mechanical strengths of the diazo resin by formulating it with another resin having good abrasion resistance. Usually, the latter method is preferred since the method of the lacquer reinforcement must be carried out in complicated steps.
The requirements for the resin to be blended with the photosensitive diazo resin are good compatibility with the diazo resin, stability of the photosensitive resin blend, large difference in the solubility in the development solution between the areas exposed and unexposed to light and high mechanical strengths as well as good ink-receptivity of the photocured resin composition on the areas exposed to light forming the image areas of the printing plate.
Unfortunately, however, none of the hitherto proposed photosensitive diazo resin compositions are satisfactory in one or more respects of the above requirements. For example, a resin composition disclosed in Japanese Patent Publication No. 52-7364 and U.S. Pat. No. 2,826,501, which is a combination of a diazo compound and water- or alkali-soluble polymer compound, is defective in the poor stability during storage and the low ink-receptivity of the photocured layer of the resin composition forming the image areas of the printing plate even though the resin composition has a good development performance, i.e. large difference in the solubility in the development solution between the areas exposed and unexposed to light.
Recently, a photosensitive resin composition suitable for the preparation of a planographic printing plate has been disclosed in U.S. Pat. No. 4,123,276. The photosensitive resin composition here proposed comprises a diazo compound and a resin which is a copolymer composed of recurring units of hydroxyalkyl acrylate or methacrylate, acrylonitrile or methacrylonitrile and an .alpha.,.beta.-unsaturated carboxylic acid such as acrylic acid or methacrylic acid.
The above disclosed photosensitive resin composition is also not free from several drawbacks. For example, firstly, the ink-receptivity of the photocured layer of the resin composition is not always satisfactory, presumably, due to the large contents of the hydrophilic groups of hydroxyl and carboxyl groups. The ink-receptivity of the resin composition can be improved by admixing another kind of resin having good ink-receptivity but admixture of such a modifier resin causes a second problem that the compatibility of the resin and the diazo compound is sometimes poor so that the coating solution of the resin composition must be prepared by use of limited kinds of organic solvents. Furthermore, the solubility of the uncured resin composition is relatively low even in the recommended development solution so that the development process takes usually as long as about 1 minute or longer.